The field of the disclosure relates generally to gas turbine engines and, more specifically, to rotor shaft architectures for gas turbine engines and methods of assembly thereof.
At least some known gas turbine engines, such as turbofans, include a core engine, a fan assembly, a low pressure compressor, and a low pressure turbine. The core engine includes a high pressure compressor, a combustor, and a high pressure turbine coupled together in a serial flow relationship. More specifically, the high pressure compressor and turbine are coupled through a high pressure rotatable shaft that is supported by a plurality of high pressure bearing assemblies to form a high pressure rotor assembly. The low pressure compressor and turbine are coupled through a low pressure rotatable drive shaft that is supported by a plurality of low pressure bearing assemblies to form a low pressure rotor assembly. Furthermore, the fan assembly is coupled to the low pressure rotor assembly through a power gearbox. Typically, the high pressure drive shaft and the low pressure drive shaft are co-axial shafts coupled in a serial relationship.
In operation, air is channeled through the low pressure compressor and high pressure compressor to increase pressure thereof. The compressed air is mixed with fuel and ignited at the combustor to generate a high energy gas stream. The high energy gas stream is channeled through the high pressure turbine to rotatably drive the high pressure compressor through the high pressure shaft. The high energy gas stream is then channeled through the low pressure turbine to rotatably drive the low pressure compressor and the fan assembly through the low pressure shaft. The power gearbox enables the low pressure shaft to rotate at a higher speed than the fan assembly, thereby, increasing efficiency of the low pressure turbine.
However, the operational rotational speed of the low pressure shaft is typically limited by a third natural frequency shaft bending mode. The low pressure shaft has natural frequencies that are in part defined by its stiffness, which is based on its geometry, material, and length. As such, during operation of turbofan engine, the low pressure shaft will vibrate at different frequencies depending in part on the rotational speed of the low pressure shaft. First and second natural frequency modes are rigid modes that induce forces into the turbofan engine that are absorbed by dampers coupled to the bearing assemblies. The third natural frequency mode, however, is a bending mode of the low pressure shaft that is excited when the rotational speed of the shaft coincides with the location of the third natural frequency. If the gas turbine engine operates at the third natural frequency mode, then bending is induced into the low pressure shaft between the bearing assemblies, increasing the induced forces of the rotor assembly with the potential of inducing rotordynamic instability therein. As such, gas turbine engines typically operate below rotational speeds which excite the third natural frequency mode of the low pressure shaft.